It is known to prepare rigid polyurethane foams by the reaction of isocyanates and polyols in the presence of blowing agents, catalysts and foam stabilizers. The addition of high water levels to standard rigid foam systems often results in an equivalent ratio imbalance and reduced processing latitude. The resultant foam tends to have a friable surface leading to poor adhesion which is particularly disadvantageous in the producing of appliances. In addition, when compared to standard foam systems, slower systems (i.e., gel times of more than 100 seconds), show significant primary shrinkage. Primary shrinkage is the shrinkage which occurs within the first 24 hours after the foam is made without subjecting the foam to any special test conditions.
A considerable effort has been expended to develop systems with both low chlorofluorocarbon levels and relatively long reactivity. When conventional systems which are acceptable under fast molding conditions are slowed down, the overall performance deteriorates. Specifically, the resultant foams become friable and pull away from the walls of the appliance within hours.
Polyols conventionally used to produce flexible foams are used extensively in elastomers and in ultra low density molded applications, such as packaging foams. A typical packaging foam formulation contains a 35 OH number triol, water, surfactant and catalyst. In all these cases, extreme care is taken to obtain a nearly total open cell content in order to prevent foam collapse and shrinkage. In the case of elastomers, they are generally prepared in such a way so as to prevent foam formation.
Typically, in the preparation of rigid, closed cell polyurethane foams, the polyols used are of a relatively high hydroxyl number (see, e.g., U.S. Pat. No. 4,092,276). U.S. Pat. No. 4,094,828 describes the preparation of a rigid polyurethane foam from a polyether polyol having a hydroxyl number of from 200 to 800 and a copolymer of allyl alcohol and styrene. The hydroxyl numbers of the only two allyl alcohol/styrene copolymers exemplified in the '828 patent are 280 and 249.
U.S. Pat. No. 4,362,823 describes the preparation of a rigid closed cell polyurethane foam wherein from about 0.1 to about 3 parts by weight per 100 parts by weight of rigid polyol, of a propoxylated propylene glycol is used as a substitute for a standard surfactant.